Our work is concerned with enzyme-mediated covalent interactions between protein chains, particularly the formation of epsilon-(gamma-glutamic)lysine crosslinks. The material we study primarily is embryonic chick heart. We are also studying the phosphorylation of certain protein chains and examining the possibility that a phosphorylated intermediate is a precursor of the crosslinking reaction. In order to carry out these studies more effectively, we are in the process of developing new assays for the process of crosslink formation and crosslink breakdown.